1. Field of the Invention
The present invention relates to a process for producing irones which have a violet-tone fragrance.
2 Description of the Related Art
Natural irones are contained in the essential oil of orrisroot in an amount of 60 to 80%, and said to be comprised of .gamma.-irone (75%) and .alpha.-irone (25%).
Several processes have been reported for the synthesis of irones, and they all comprise, at their final step, conducting the ring-closure of pseudoirones. ##STR1##
In the above reaction, there have been used as the ring closing agents Bronsted acids such as sulfuric acid and phosphoric acid and Lewis acids such as boron trifluoride and tin tetrachloride.
It is known that the ring closing of pseudirone gives a mixture of the following 3 types of irones. ##STR2##
Among the above, the .alpha. forms generate the fragrance of natural irones most representatively, while that of the .beta. form is said to be more close to those of ionone and methylionone [Osamu Okuda, Koryo Kagaku Soran, Vol. 2, page 970 (Hirokawa publishing Co.)]. It is also said that, among the .alpha. forms, .alpha.-cis has better fragrance [Fragrance Journal, No. 17, page 72 (1976)].
It is also known that the ring closing of pseudoirone by the use of a mineral acid such as sulfuric acid or phosphoric acid gives the .alpha.-trans form preferentially to the .alpha.-cis, while the use of boron trifluoride or tin tetrachloride leads to formation of the .alpha.-cis form in preference to the .alpha.-trans. See, for example Chemical Abstract, 17833f (1957). Boron trifluoride and tin tetrachloride however are expensive and moreover, the use of tin tetrachloride raises the problem of waste water pollution. Thus, there has been established no process for the commercial production of the .alpha.-cis form in a high yield.
Japanese patent Application Laid-open No. 258773/1990 discloses a process for producing a cyclic terpene which comprises conducting ring-closing reaction in the presence of chlorosulfonic acid as a ring-closing agent. The cyclic terpene desired by this known reaction however contains no isomers, and hence the reaction is fundamentally different from the reaction for obtaining irone.